On digestion of wood or other vegetable material in order to produce pulp (e.g. paper pulp) according to the sulphate method the pulping chemicals are recovered in the following way:
The waste liquor obtained during the digestion of wood, the black liquor, is evaporated to give concentrated waste liquor, thick waste liquor, that is burnt in a soda recovery boiler. A smelt is obtained that is led in smelt spouts down in a tank where it is dissolved in weak liquor obtained by the washing of lime sludge in the causticizing department. Green liquor is then obtained. It is led to a green liquor clarifier for the removal of solid particles by sedimentation. The clarified green liquor is led to a lime-slaker where caustic lime (calcium oxide) is fed to it. The calcium oxide will react with the sodium carbonate in the green liquor and a solution of sodium hydroxide and a precipitate of calcium carbonate, lime sludge or mud, as it is called, is formed. The lime sludge is separated by filtration from the white liquor obtained by the reaction (causticizing). The lime sludge is washed with hot water in order to take care of soluble alkaline chemical compounds. The weak liquor from this process is led to the above mentioned tank for dissolution of the smelt. The lime sludge is reburnt in a rotating lime kiln to give caustic lime that is brought back to the lime-slaker for causticizing of the green liquor.
In order to avoid an increase of the concentration of inert compounds in the white liquor and in the lime-cycle, it is necessary to draw off a small quantity of caustic lime or lime sludge. The amount of lime or lime sludge that must be extracted from the process system is partly dependent on how well the clarifying of the green liquor works and is normally 3-6% of the charged amount of caustic lime, i.e. 7.5-15 kg per metric ton pulp.
The smelt obtained in the soda recovery boiler contains besides the sodium- and sulphur-compounds necessary for the digestion process also small amounts of inorganic, sparingly soluble compounds of silicon and aluminum, which are incrustation forming and hence have to be removed from the process, and also variable amounts of carbon particles (soot) which are considered as rendering the cleaning of the green liquor more difficult. These impurities follow the green liquor as more or less fine particles. They are very difficult to separate and have up to now only been able to be removed by settling in so called green liquor clarifiers and then only with very low surface load, for example, about 0.5 m/h. The result of the cleaning varies normally very much and the concentration of the remaining impurities in the green liquor is normally not below 50 mg/l.
As very large volumes of green liquor must be cleaned, i.e., about 3.5 m.sup.3 per metric ton pulp, very large clarifiers are required in order to give acceptable cleaning. It has consequently been a object desired to be able to remove the impurities on conventional filters as exemplified by press filters, vacuum filters, disc filters, and drum filters, partly in order to decrease the space needed and the investment costs and partly in order to improve the cleaning of the green liquor. This has, however, not been possible because the impurities cause large pressure drops in conventional filters and consequently a very low capacity.
Attempts to filter the green liquor have, however, been performed. For example, in the Swedish patent application 8103333-4 a process is described according to which lime sludge is laid as a filtering layer on a press filter before starting the filtration of the green liquor. The practical effect of this method of apporach depends among other things on how much of the impurities in the green liquor the filtering layer can take up before it is filled up and must be replaced with a new layer. This procedure has not yet been applied commercially.